Optical fibers are widely used within telecommunication systems for high-speed information transfer, particularly in the core or backbone network. Central to next generation telecommunications network and the roll out of broadband in e.g. the UK, is an all-optical fiber network throughout the core network which reaches out to customers (corporate users, enterprises, service providers) and the local distribution points that serve residential premises. In time, optical fibers may also extend to all residential premises in the form of an optical fiber extending through the access network to the cabinet, the curb or directly into the premises (FTTx) to the millions of end customers in e.g. the UK.
This entails a major program to install optical fiber which is underway across the UK, involving millions of kilometers of fiber in the field, particularly in the local access which to date has been predominantly a copper domain. The installation method of choice is blown fiber, a technique originally described in applicant's seminal patent EP 108590 and now in widespread commercial use. In short, this involves the installation of a, typically lightweight, optical unit, cable or fiber into a pipe or tube by use at least in part of the viscous drag effect produced by the flow of gaseous medium along the bore of the tube. The gaseous medium is chosen to be compatible with the environment in which the method is performed, and in ordinary environments will be a non-hazardous gas or gas mixture. With the proviso about compatibility with the environment, the gaseous medium is preferably air or nitrogen.
The blown fiber process begins with drawing in a tube (also referred to as a duct or microduct) typically of a plastics material, or more usually a bound and sheathed bundle of tubes, along existing or new underground ducts in the field. Each tube has an inner diameter of typically 2.5-3.5 mm (although larger diameters are also used, commensurate with the size of cable to be installed) which can accommodate a fiber cable containing one fiber or a tight bundle of several fibers. These tubes are laid as a single continuous span between convenient access points such as surface boxes and inspection chambers (manholes) which typically might be hundreds of meters apart in urban settings, reducing to 100 m or less in business districts or like areas. Spans like this are laid in a series along the route that the fiber cable must be laid. When a telecommunication connection is required at a location at or along the fiber cable route, a fiber cable is installed in each span by blowing it down the tube from one end. Alternatively the cable could be blown down a concatenation of spans in one step if circumstances permit. This is repeated for each span, or concatenation of spans, until a continuous fiber path has been laid between the end points of the whole route.
It is crucial to choose the correct tube path at the head end during installation, so that the fiber unit emerges at the desired remote, destination end. During installation however, the operator at one of the installation points would typically be presented with a large number of confusable conduit or tube openings, each representing a path leading to a potentially different destination. The tube openings at each end are usually mapped to their destinations e.g. by color-coding. If however the tube openings are wrongly mapped, or the records otherwise inaccurate, mistakes can result in fibers being blown down the wrong path, with a consequent need perhaps to recover the mis-blown fiber, and the need subsequently to identify the correct conduit end for the desired installation path. This is especially so if the working conditions are poor e.g. in adverse weather or down a manhole or in poor lighting.
Where the path comprises a number of tube lengths linked together by intermediate connectors, yet another problem may lie in broken or incorrect connections between lengths of conduit tubes within the network, so that the fiber unit may get lost within the system during installation and never emerge at the desired destination. Yet another issue may be the possibility that the fiber unit, during installation, could be impeded by an imperfect connection or a tight bend or some other source of friction in the tube, and again never emerge at the chosen destination.
For any of these or other reasons, the fiber unit may, during installation, emerge in the wrong place, or in an extreme case, not at all. Add to that some uncertainty about the exact length of the tube route down which the fiber unit is being installed, so that the operator may not even know in a timely manner when something has gone wrong.
Currently these problems are avoided as far as possible through the use of two engineers or operators during installation, one at each end of the installation tube path. The operator at the head end of the tube path is in charge of the installation process in that he controls the fiber cable ingress apparatus (known as a blowing head which is described in general in e.g. WO88/00713, or the like) and the supply of compressed gas, e.g. a compressor. This head end operator feeds the fiber cable into the tube requiring population in the direction of the second operator located at the remote end of the installation path (i.e. the desired destination). The second operator signals back to the operator at the head end, typically using a walkie-talkie, mobile phone or the like, to confirm that installation gas has arrived at the destination end, before the first operator starts installing the fiber cable into the waiting tube. Upon arrival of the advancing end of the fiber cable at the far end of the tubular path, the second operator signals confirmation of this fact back to the first operator who then concludes the installation process.
The second operator is typically required because the operator at the head end is unable to know the status of the remote end during an installation—since the remote end is typically many tens or hundreds of meters away and is anyway unlikely to be visible to the head end operator.
Consequently, the installation process is relatively labor-intensive, requiring two operators to work on a single installation. This has an impact on the overall cost of optical fiber installation, a problem now especially significant in the FTTP context with scale and considerable installation volumes involved.
Various methods requiring only a single operator installation of blown fiber have been developed, to obtain a significant saving in manpower and hence cost. In the simplest method, the length of the conduit route is known, allowing the operator to know that the fiber has (probably) arrived at the remote end when the required length of fiber unit has been played out. This relies on the map record of conduit route being up to date and accurate, and presumes a smooth and obstruction-free conduit route. Neither of these can be guaranteed in practice.
Another known practice is to install at the remote end of the conduit a barrier of porous material such as an “airstone” which is constructed of a porous material which allows gas through but which will stop further progress of the fiber unit. The airstone is temporarily placed at the mouth of the destination remote end of the tube conduit. When the fiber ceases to travel down the tube, this is an indication that the far end of the fiber may have reached the destination end and has been retained by the airstone barrier. However, lack of further progress is ambiguous as to whether the fiber unit has indeed reached the porous airstone at the destination end, or if instead the fiber unit is caught on an obstruction at some intermediate point along the length of the conduit.
These, together with other methods like those described in WO9103756 or WO/9812588, also describe how fiber arrival can be detected by the single operator at the head end of the installation; the initial step of detecting that the compressed gas fed into the head tube end is not addressed in the above techniques.
One method, developed by the applicants of the present case and described in PCT application GB2008/004284, describes a gas flow and fiber cable sensing methodology using a whistle arrangement. When compressed gas is fed into the correct tube at the head end, it will eventually flow through the remote end of the tube to which is attached a device including the whistle arrangement, which produces an acoustic status signal indicating gas arrival When the fiber cable subsequently arrives at the remote end it interacts with the resonant cavity of the whistle arrangement device to cause the frequency of the sound produced by gas flow over or through the whistle arrangement to change (in tone or to cease). The status signal and any changes thereto are detected at the remote end, where the detector is either physically or operatively coupled to the device at the remote end, and then conveyed to the engineer at the head end over a conventional radio channel. This shortcoming in the methodology may affect its adoption for wide-scale deployment, as the availability and resilience of that channel cannot be guaranteed in a field setting, owing to the tube ends being routinely located below ground, which adversely affects the transmission and reception of the status signal at the head end. Furthermore, the electronic circuitry (signal amplifier, signal processor, radio transmitter, etc) necessarily co-located with the whistle arrangement device at the remote location typically requires a local power source, such as a battery, which raises maintenance issues.